This proposal is aimed at understanding how bacteria interact with their eukaryotic hosts and how the availability of nutrients affects these interactions. The bacterium Xenorhabdus nematophilus provides a valuable model system of host microbe interactions: it is a symbiont that resides in the intestine of the nematode Steinernema carpocapsae and a pathogen that kills larval stage insects. The goals of this research are to identify the molecular mechanisms used by X. nematophilus to mediate host interactions. Nutrient deprivation in many Gram-negative bacteria triggers development of increased resistance to stress and the production of virulence determinants. This response is partially regulated by a specific sigma factor, ss. The first specific aim of this proposal is to analyze the role of ss and ss-dependent gene expression in the symbiosis and pathogenicity of X. nematophilus with its eukaryotic hosts. The results of this work will begin to address the genetic regulation of survival and adaptation in X. nematophilus and will provide insight into how the availability of nutrients affects host-bacterium interactions. X. nematophilus is likely to have evolved multiple regulatory mechanisms, including the starvation response, that mediate host interactions. While specific Aim 1 focuses entirely on the starvation response, the goal of Specific Aim 2 is to perform genetic tests to identify additional bacterial regulatory mechanisms that mediate symbiosis between X. nematophilus and the nematode. The goal is to obtain and characterize X. nematophilus mutants that are no longer retained within the intestinal vesicles of nematodes. This work will provide insight into the genes and gene products that are essential for symbiosis and will aid in the elucidation of the regulation of host interactions in X. nematophilus and other bacteria.